A drilling system comprising a percussive shearing drill bit is known and described in U.S. Pat. No. 6,253,864. FIG. 4 of said US patent depicts a percussive shearing bit having a unitary body, a means for attachment to a drill string, and a plurality of blades housing a plurality of shear cutting elements. Fluid outlets are situated on the head of the unitary body between the blades. The blades consist of a series of receptacles to house the shear cutting elements and a shelf that runs along each blade before the cutting elements. The shelf serves to direct cuttings away from the operative surface of the bit.
In operation, the known percussive shearing drill bit is rotated about its longitudinal axis shearing off the rock formation as the drill bit rotates. A hammer simultaneously impacts the bit thereby providing an additional percussive drilling force. The shear cutting elements have been specially designed to withstand the unusual stresses induced by combined percussive/shear drilling, in that a distal portion of the shear cutter has been rounded to prevent large localised stresses in the cutters. Thus in the shear cutting elements a compromise is found for both shearing and axial cutting.
The known drilling system has been found to suffer from the risk of stick-slip torsional vibrating during drilling of certain types of earth formations. When this occurs, the bit is captured to a standstill into the earth formation while the drill string is twisted by the surface rotary drive until it abruptly releases with relatively high rotational speed. Such a stick-slip torsional vibration repeats periodically and the high rotational speed associated with the stick-slip torsional vibration can severly damage the cutters on the drill bit.